Objection 1. It would seem that happiness consists in an act of the will. For Augustine says (De Civ. Dei xix, 10,11), that man's happiness consists in peace; wherefore it is written (Psalm 147:3): "Who hath placed peace in thy end [Douay: 'borders']". But peace pertains to the will. Therefore man's happiness is in the will.
Objection 2. Further, happiness is the supreme good. But good is the object of the will. Therefore happiness consists in an operation of the will.
Objection 3. Further, the last end corresponds to the first mover: thus the last end of the whole army is victory, which is the end of the general, who moves all the men. But the first mover in regard to operations is the will: because it moves the other powers, as we shall state further on (9, 1,3). Therefore happiness regards the will.
Objection 4. Further, if happiness be an operation, it must needs be man's most excellent operation. But the love of God, which is an act of the will, is a more excellent operation than knowledge, which is an operation of the intellect, as the Apostle declares (1 Corinthians 13). Therefore it seems that happiness consists in an act of the will.
Objection 5. Further, Augustine says (De Trin. xiii, 5) that "happy is he who has whatever he desires, and desires nothing amiss." And a little further on (6) he adds: "He is most happy who desires well, whatever he desires: for good things make a man happy, and such a man already possesses some good--i.e. a good will." Therefore happiness consists in an act of the will.
On the contrary, Our Lord said (John 17:3): "This is eternal life: that they may know Thee, the only true God." Now eternal life is the last end, as stated above (02, ad 1). Therefore man's happiness consists in the knowledge of God, which is an act of the intellect.
I answer that, As stated above (Question 2, Article 6) two things are needed for happiness: one, which is the essence of happiness: the other, that is, as it were, its proper accident, i.e. the delight connected with it. I say, then, that as to the very essence of happiness, it is impossible for it to consist in an act of the will. For it is evident from what has been said (1,2; 2, 7) that happiness is the attainment of the last end. But the attainment of the end does not consist in the very act of the will. For the will is directed to the end, both absent, when it desires it; and present, when it is delighted by resting therein. Now it is evident that the desire itself of the end is not the attainment of the end, but is a movement towards the end: while delight comes to the will from the end being present; and not conversely, is a thing made present, by the fact that the will delights in it. Therefore, that the end be present to him who desires it, must be due to something else than an act of the will.
This is evidently the case in regard to sensible ends. For if the acquisition of money were through an act of the will, the covetous man would have it from the very moment that he wished for it. But at the moment it is far from him; and he attains it, by grasping it in his hand, or in some like manner; and then he delights in the money got. And so it is with an intelligible end. For at first we desire to attain an intelligible end; we attain it, through its being made present to us by an act of the intellect; and then the delighted will rests in the end when attained.
So, therefore, the essence of happiness consists in an act of the intellect: but the delight that results from happiness pertains to the will. In this sense Augustine says (Confess. x, 23) that happiness is "joy in truth," because, to wit, joy itself is the consummation of happiness.
Reply to Objection 1. Peace pertains to man's last end, not as though it were the very essence of happiness; but because it is antecedent and consequent thereto: antecedent, in so far as all those things are removed which disturb and hinder man in attaining the last end: consequent inasmuch as when man has attained his last end, he remains at peace, his desire being at rest.
Reply to Objection 2. The will's first object is not its act: just as neither is the first object of the sight, vision, but a visible thing. Wherefore, from the very fact that happiness belongs to the will, as the will's first object, it follows that it does not belong to it as its act.
Reply to Objection 3. The intellect apprehends the end before the will does: yet motion towards the end begins in the will. And therefore to the will belongs that which last of all follows the attainment of the end, viz. delight or enjoyment.
Reply to Objection 4. Love ranks above knowledge in moving, but knowledge precedes love in attaining: for "naught is loved save what is known," as Augustine says (De Trin. x, 1). Consequently we first attain an intelligible end by an act of the intellect; just as we first attain a sensible end by an act of sense.
Reply to Objection 5. He who has whatever he desires, is happy, because he has what he desires: and this indeed is by something other than the act of his will. But to desire nothing amiss is needed for happiness, as a necessary disposition thereto. And a good will is reckoned among the good things which make a man happy, forasmuch as it is an inclination of the will: just as a movement is reduced to the genus of its terminus, for instance, "alteration" to the genus "quality."
Source: Summa Theologica (New Advent)